To date, it has not been possible to rapidly and quantitatively solubilize and recover chemical components from lignocellulosic materials on an economic basis. The literature describes various organosolv processes for delignification and saccharification of lignocellulosic materials and vegetable crops. In general, such processes involve the use of a mixture of water and a solvent such as an alcohol or a ketone of a limited polar nature along with an acidic compound to encourage the hydrolysis action. Known processes have been characterised by poor delignification ability, slow hydrolysis rates and extensive sugar conversion into undesirable non sugars, mainly furfurals and organic acids. Condensed lignins obtained from such processes are of a highly condensed form and are not usually suitable for chemical processing.
U.S. Pat. No. 1,856,567, Kleinert et al., 1932, teaches the use of aqueous alcohol at elevated temperatures for production of cellulose pulp in a pressure vessel using small quantities of acids or bases as delignification aids. Treatment is described in terms of three hour steps, which is too long for commercial exploitation.
Conventional hydrolysis processes for delignification and saccharification of lignocellulosic materials using water and acid suffer the following drawbacks.
1. Most lignin is not solubilized and is further condensed to a solid mass that cannot be used for the manufacture of chemicals. This mass adds a complication in that it absorbs sugars dissolved in the liquor. The sugars can be removed only by extensive washing.
2. During the process there is virtually no delignification reaction. This reduces the accessibility of the liquor to the carbohydrate and hence reduces the rate of hydrolysis.
3. The hydrogen ion has a high affinity to the polar water molecule. This reduces the catalytic effect of the hydrogen ion on the carbohydrate.
It is believed that utilizing an organic solvent of low affinity to a hydrogen ion together with water and acid at high temperature and pressure facilitates simultaneous dissolution of lignin and carbohydrate, and intensifies the catalytic effect of the acid. This process overcomes many drawbacks of the conventional hydrolysis process. However, the much increased dissolution rate of wood components is offset somewhat by a relatively short working time available to compete with the decomposition of the dissolved compounds to undesirable degraded products.
Rapid removal of the desirable dissolved compounds would provide a method of minimizing the undesirable decomposition effect. This can be easily done in a batch process. But to date, there have been many difficulties to overcome in developing a method to provide continuous rapid removal of dissolved compounds before appreciable degradation takes place. Some of the difficulties to be overcome have been that a large excess amount of expensive liquor is required to promote a complete reaction and high energy cost is incurred in recovering the liquor products through evaporation.
U.S. Pat. No. 4,409,032, wherein I am one of the inventors, describes a process for continuous saccharification and delignification of wood using acetone and water mixture with small amounts of acid and rapid cooling after saccharification. Rapid cooling is used to prevent degradation of the saccharification products. My copending U.S. application Ser. No. 347,238, filed Feb. 9, 1982 now U.S. Pat. No. 4,470,851 describes a related process using higher proportions of acetone. This U.S. application and patent are related to Canadian application Ser. No. 395,820, filed Feb. 9, 1982 and Canadian Patent No. 1,100,266. These methods work very well but improvement was needed.